Multilayer thick film circuits have been used for many years to increase circuit functionality per unit of area. Recent advances in circuit technology have placed new demands on dielectric materials for this use. Heretofore, most of the dielectric materials used in multiple circuits have been conventional thick film dielectric compositions. These are comprised of finely divided particles of dielectric solids and inorganic binders dispersed in an inert organic medium. Such thick film materials are usually applied by screen printing, though they may be applied by other means as well. Thick film materials of this type are very important and will continue to be so.
In constructing a multilayer circuit using thick film materials, it is necessary to print, dry and fire sequentially each functional layer before the next layer is applied. Thus, in a typical situation involving multilayer circuits having, say, ten layers, thirty separate processing steps (printing, drying and firing each layer) are required as well as ten inspections to assure the quality of each of the processed layers. Such a complex process is, of course, expensive both because of the great number of steps and because of the high yield losses which are normally incident to such a complex procedure. Moreover, the present state of the art limits these structures to a maximum of ten layers.
Another approach to this problem has been the use of dielectric tapes, which are thin sheets of ceramic dielectric material, such as alumina. The tape process involves lamination of a number of sheets of unfired tape (usually alumina) interspersed with alternating printed layers of electrically functional material. However, very high temperatures (e.g. on the order of 1600.degree. C.) are required to sinter the alumina. Thus, only very high melting electrically functional materials such as tungsten and molybdenum can be used. Unfortunately, molybdenum and tungsten have poor electrical and, in particular, conductivity properties which make them less satisfactory for very high speed, highly complex circuitry.
Recently, low temperature co-fired (LTCF) technology has been introduced as a method for fabricating multilayer circuits. This technology offers the combination of the processing advantages of the alumina-based tape technology plus the materials advantages of thick film technology. The LTCF tape systems have firing temperatures below 1000.degree. C. and allow the use of highly electrically functional metals such as silver, palladium/silver, gold, platinum, nickel and copper.
One type of dielectric tape system, as disclosed in commonly assigned, copending U.S. application Ser. No. 07/423,367, filed concurrently herewith, uses amorphous crystallizable glass plus glass frit to achieve tapes with low dielectric constant (K), dimensional stability, low dielectric loss and variable thermal coefficient of expansion (TCE). However, this system suffers the disadvantage of requiring relatively long firing times in order to develop the degree of crystallinity necessary to become rigid for subsequent distortion-free refiring. Such long firing times cannot be realized in the conventional belt furnaces commonly used throughout the thick film industry. Therefore, the copending dielectric compositions require batch processing in box furnaces, which is an economic disadvantage.
In addition, in these copending compositions, the amorphous borosilicate glass (frit) tends to migrate to the surface during firing unless composition, firing time and firing temperature are carefully matched within a narrow range. Glass migration to the surface causes poor adhesion of the electrically functional metallized layer to the underlying structure which can lead to undesirable electrical shorts when portions of the metallized layer fall off the dielectric layer. It is difficult to tailor the compositions of the copending application to a particular TCE and minimal firing temperature without glass migration.
Thus, there is a need for improved dielectric compositions which can reliably produce fired parts with predetermined electrical properties. There is also a need for dielectric compositions for which the required firing time is short enough to allow for processing in conventional belt furnaces. And there is a need for dielectric compositions which have variable TCE's such that the TCE can be controlled to match the TCE of other materials such as silicon, gallium arsenide and alumina.